If you want to inexpensive, you can try the harbor freight tail pipe cutter. Pretty decent reviews. Might be worth checking out if you have a HF local.
This is what I tried the first time around and found it to be useless. Mine may have been defective though. They did however, take it back and give me a refund with no questions asked.
I've used the HF exhaust pipe cutter many times. It's a bit tricky to get started off cutting straight. You have to ensure it's nice and perpendicular to the can, and once you get all the wheels tracking on the same line you have to take it slow with the cutting until it makes a distinct groove.